In numerous applications, there is a need to precisely measure the position of an object. For example, many manufacturing processes require precise positioning of a moving stage. Several techniques for determining the position of a moving object have been developed. Some of these techniques are discussed below.
In some prior art optical mouse devices, a random two-dimensional pattern consisting of the surface irregularity of a mouse pad is captured continuously by a digital camera into sequential N×N pixel-arrays. Memory depth is at least one past the current array. Nine two-dimensional correlations (e.g., one step in each direction: left, right, up, down, both directions of two diagonals, plus “no-change”) are computed. By observing the maximum of the nine correlations, the direction of the mouse travel can be determined. By interpolation of the correlation values, the length of travel can be estimated to a fraction of a sensor pixel. This method for determining location is incremental and not absolute, and travel between frames typically cannot exceed one pixel, or errors may result.
In some other prior art applications, specific targets are used instead of random targets. Some one-dimensional incremental encoders, for example, use two gratings in quadrature of a one-dimensional sinusoid to produce two continuous image grey-scales. Calibration of the known intensity profile of the scale enables one to interpolate to less than one cycle, and counting the cycles in the proper travel direction made possible by two signals in quadrature allow determination of both coarse location and fine location. This approach, very commonly used in industry, is also an incremental measurement technique just as with the mouse, and subject to the same limitations. To use one-dimension encoder technology for two dimension applications generally requires cumbersome mechanical stage stacking.
In some other prior art applications, to reduce the need for making a two-dimensional correlation for every N×N pattern over the entire target for two-dimensional absolute position encoding, position labeling codes have been used. The codes are periodically interleaved into a random or periodic fine grey-scale background. The (yet unknown) position code or codes are first identified and extracted from the image, and their contents are then deciphered. The location of the code within the image is then found more accurately, and by inference, the image location is established.
Several prior art methods interpolate between discrete correlations by observing the analog shape of the correlation functions. Cross-correlation of the target pattern reduces the accuracy of this approach and must be carefully controlled.
Prior art systems for determining position, such as those described above, are typically not very efficient, require relatively large amounts of memory, and the targets used in some of these systems are complex to make and use.